perf: make module downward search linear

[OskarEichler]

Summary

• replace repeated queue shift() calls with an advancing index
• preserve breadth-first traversal order and search limits

Why

Downward node-module discovery can explore up to 2,000 directories. Removing from the front of the array shifts every remaining entry, turning queue management into quadratic work.

Tests

• TEST_FILES=moduleManagerTest corepack pnpm ci:test (22 passing)
• corepack pnpm compile
• corepack pnpm ci:validate
• git diff --check

Breaking changes

None.

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Latest commit: 29178c0

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